1. Field of the Invention
The present invention relates to a turboprop propulsion apparatus for an aircraft.
2. Background Art
Turboprop engines are commonly designed to drive either a single row of propellers or two rows of counter rotating propellers, and the propeller(s) can be mounted forward of the engine (called the "tractor" installation) or rearwardly of the engine (called the "pusher" installation). The advantage of the pusher system over the tractor arrangement is that in the pusher arrangement, the engine has an efficient free stream inlet, and the high speed propeller jet does not impinge on airplane or nacelle surfaces, thus avoiding scrubbing drag. Also, a pusher engine installation on the aft body of an airplane avoids the cabin noise problems that wing mounted tractor nacelle engines may cause. However, there is a major drawback of the pusher arrangement in that it is difficult to find a convenient location for the gas turbine exhaust.
There are a number of possibilities to alleviate the problem of the gas turbine exhaust in the pusher arrangement. What would appear to be a relatively simple solution would be to build an exhaust upstream of the propellers. This exhaust could be annular or lobed. However, for either case, the nacelle skin downstream of the exhaust and the lower part of the propeller blades would have to be designed for elevated temperatures caused by the hot exhaust gases. Also, these components may also have to be built from fireproof materials to withstand a brief fire during a wet start.
Another possibility for the primary exhaust is to duct the flow through a mount strut and discharge it at another location (e.g. through the fuselage of the airplane at an aft location). However, this solution involves the weight and cost of a long steel duct, possibly insulated, and the performance loss associated with the pressure drop in a long, elliptical duct. Further, the engine nacelle would have a relatively long configuration, since the elements would normally be arranged in series (these elements being the inlet, gas generator, power turbine, exhaust collector, gear box, and propeller(s)).
A third possibility for a turboprop engine for counter-rotating pusher propellers is to use two counter-rotating, direct drive free turbines, with the propellers mounted over the turbine drums and a plug-type primary exhaust nozzle at the aft end of the nacelle. This makes a rather compact (although rather thick) nacelle, and the primary exhaust is properly located. However, there are several drawbacks. First, there is the complexity of the two multi-stage, counter-rotating free turbines. Also, there is low turbine efficiency resulting from the low turbine blade rotational speeds and the high number of stages needed to extract the required power. Then the propeller efficiency is compromised because of the high disc loading and propeller tip speed. Further, there is the potential noise problem resulting from the high propeller disc loading and tip speed.
Contrary to the pusher engine installation, tractor turboprop engine installations have no exhaust problem, but inlet problems. This is particularly true for the in-line arrangement commonly used for high performance airplanes. In the in-line arrangement, the engine, the gear box and the propellers are all arranged along a common centerline. The power turbine at the rear end of the engine drives a power shaft through the center of the engine. The reduction gearing and the propellers are forward of the gas generator. In order to provide an air inlet for the gas generator, the reduction gearbox has to be located well forward of the engine face. This makes for a large overhang of the gearbox and propellers with the associated structural weight penalties. The engine inlet in this form of a tractor arrangement is either an S-shaped scoop inlet or a curved annular inlet. Both of these inlet types have high inlet pressure losses and cause high inlet pressure distortions.
In general, both the pusher and tractor configurations of a turboprop engine have problems relative to the total gaseous flow into, through and from the engine.
A search of the patent literature has disclosed a number of patents relating to turboprop engines or the like. These are as follows.
U.S. Pat. No. 2,478,206--Redding shows a turboprop engine where there are two counter-rotating propeller rows mounted directly to counter-rotating turbine blades.
U.S. Pat. No. 2,504,414--Hawthorne shows a turboprop engine where there are two counter-rotating blade rows driven from two separate turbine sections. In another embodiment, the counter-rotating blade rows are driven from counter rotating turbine members in the same turbine section.
U.S. Pat. No. 2,505,660--Baumann discloses a turbine engine where air propelling blades are driven directly from rotating turbine portions.
U.S. Pat. No. 2,526,409--Price discloses in FIGS. 2 and 8 a gear drive system for driving counter-rotating propellers. There is a stationary spider on which a plurality of planet gears are mounted, with the turbine driving the planet gears about stationary axes of rotation. The planet gears engage an outer ring gear which rotates one set of blades in one direction, and the planet gears also rotate an inner gear which is connected to the second counter-rotating propeller. In FIG. 3 of that patent, there is a plurality of sets of radial flow turbine blades attached to prospective propeller blades.
U.S. Pat. No. 2,526,941--Fishbein shows a gas turbine system for an aircraft where counter-rotating propellers are driven from a gear box.
U.S. Pat. No. 2,541,098--Redding shows a gas turbine where the propeller is driven from the turbine section.
U.S. Pat. No. 2,663,517--Price shows a particular type of mounting structure for a turboprop engine.
In U.S. Pat. No. 1,663,749--Price,there is shown a gas turbine engine having counter-rotating propellers. There is a gear drive positioned between high and low pressure compressor sections. This gear drive, in addition to drivingly innerconnecting the two compressor sections, also has radially extending shafts which rotate outer pinion gears which in turn rotate a set of axially extending shafts 46. These shafts 46 act through gears at the ends of the shaft, one set of gears being connected to an outwardly positioned ring gear, and another set of gears being connected to an inwardly positioned ring gear. These two ring gears are each in turn attached to a respective set of propeller blades to cause the counter-rotation of the two sets of blades.
U.S. Pat. No. 2,702,985--Howell shows a turbine engine where the compressor section has two sets of compressor blades, both of which are rotatably mounted. One set of compressor blades is connected to a power turbine, while the second set of compressor blades is connected to a propeller. It would appear that in the embodiment of FIG. 1, the propeller is driven by the airflow produced by the blades powered directly from the turbine section, although the patent does not describe this in any great detail. In other embodiments, the propellers or blades are driven directly from the turbine.
U.S. Pat. No. 3,153,907--Griffith shows a power plant where different sets of propeller blades are driven from two separate engines.
French Pat. No. 934,469 (a translation of which is not presently available) shows a variety of duel propeller and single propeller arrangements in a turboprop engine.
By way of further background information, it is believed that a Russian turboprop aircraft, called the "Bear" has used a planetary gear system in connection with a turboprop engine having counter-rotating propellers. However, it is not known whether this is prior art with regard to the present invention, and the details of the construction of such an engine are not known to the applicant. However, this is mentioned to insure that the applicants are complying with their duty to disclose all potentially relevant prior art.